1. Field of the Invention
This invention relates to an engine control and combustion quality detection system for an internal combustion engine and also to a method of detecting combustion quality in such an engine.
2. Discussion of the Prior Art
Knock in an internal combustion engine causes damage to the engine and a reduction in power output. In spark ignition engines, differences between engines and the build up of deposits cause the spark advance angle at which knock occurs to vary considerably between engines. It is therefore desirable to provide a system which can detect knock accurately.
It is already known to use a knock sensor for detecting knock. Such a knock sensor may take the form of a piezo-electric sensor mounted on a cylinder block or a pressure sensor mounted inside an engine cylinder. These prior art systems suffer from problems of identifying the level of sensor output which corresponds to noise rather than to a genuine knock signal. Various measures are used to distinguish the genuine knock signal from the noise signal, and these include bandpass filtering to select the characteristic knock frequency, gating the output of the knock sensor so as to select the region in which knock occurs, and making the knock threshold adaptive. However, none of these measures provides a satisfactory solution to the problem of identifying the genuine knock signal.
In U.S. Pat. No. 4,454,750, the output of a knock sensor is compared with a background reference signal. In order to obtain the background reference signal, the sensor output is sampled and the resulting signal is averaged. With the intention of removing the knock component from the background reference signal, sampling is controlled by comparing the output of the knock sensor with a threshold which is established as being proportional to the background reference signal. However, this system suffers from the disadvantage that an error in establishing the threshold for sampling will result in an error in the background reference signal.
In European Pat. No. 155,680, a spark ignition engine is provided with a knock sensor and the output of the knock sensor is compared with a threshold. At periodic intervals, the threshold is recalibrated by gradually advancing the spark advance angle until there is a sudden increase in sensor output and then judging this point to be the knock threshold. This system suffers from the disadvantage that the gradual advancing of the spark ignition angle is not compatible with normal driving conditions.
A similar problem occurs in measuring engine roughness. When measuring engine roughness, a problem is caused by background noise due to road irregularities and difficulty in distinguishing genuine engine roughness from engine speed fluctuations caused by these road irregularities.
The term "knock" refers to high frequency fluctuations in cylinder pressure in one combustion event. "Engine roughness" refers to fluctuations in engine speed between successive ignition cycles. In this specification, the term "combustion quality" means "knock" or "engine roughness."